The present invention relates to a wire-cut discharge machine for machining a workpiece with electrical discharges caused to be occurred between a wire electrode and a workpiece.
2. Description of the Related Art
Conventionally, when a workpiece is subjected machining with such a wire-cut discharge machine, the workpiece is fixedly mounted by the aid of jigs on the upper surface of a movable table. Machining of the workpiece is performed by electric discharges intermittently occurring between the wire electrode and the workpiece. A problem with such a wire-cut discharge machine is that the wire electrode is cut or broken during machining caused by improper setting of machining conditions such as discharge energy, discharge frequency or the like.
To resume machining of the workpiece from the point where the wire is broken, it is necessary that a new wire electrode be passed through an aperture located at the machining start point on the workpiece. So the workpiece is moved back so that the point on the workpiece where the wire electrode is broken is positionally in coincidence with the machining start point.
Japanese Patent Publication KOKOKU No. 2-27089 proposes an automatic returning device for returning the workpiece to the machining start position. Machining start position coordinates representative of the machining start position of the wire electrode relative to the workpiece are determined in advance. When the wire electrode is broken during machining of the workpiece, the present position on the workpiece where the wire electrode is broken is stored in a memory as present position coordinates. Computations are performed to obtain differences in both X-axis coordinate and Y-axis coordinate between the machining start position and the present position. Based on the thus computed X-axis and Y-axis coordinate differences, the workpiece is moved back along a straight line from the present position to the machining start position.
However, if the jigs for fixing the workpiece extends across the straight line to bother the workpiece from returning to the machining start position, the jig is liable to abut against a nozzle which ejects working liquid toward the machine position of the workpiece and damage the nozzle.